1. Field of the Invention
The present invention relates to the field of energy devices, such as energy conversion devices and energy storage devices. The devices can include a porous support member having pore channels, and one or more active material layers coaxially disposed within the pore channels. The devices can be utilized in a variety of applications including, but not limited to, energy conversion applications such as photovoltaics, photocatalysis, radiation detection and light emission, and energy storage devices such as capacitors and batteries.
2. Description of Related Art
There are many useful applications of energy conversion devices, i.e., devices that convert one form of energy (e.g., light or radiation) to another form of energy (e.g., electricity). By way of example, energy conversion devices may include photovoltaic cells, radiation detectors, photocatalysis devices, light emitting diodes and other lighting devices, and chemical or biological sensors. Similarly, there are many useful applications of energy storage devices such as electrical capacitors, batteries and the like.
One of the most promising and important areas of energy conversion technologies is the conversion of solar energy into electricity using photovoltaic (PV) materials. Most PV devices are formed using a semiconductor p-n junction for energy conversion. On absorption of light, electrons are excited into the conduction band and flow from a p-type semiconductor material to an n-type semiconductor material, while the holes in the valence band flow in the opposite direction. The calculated maximum conversion efficiency for an optimal p-n junction PV device with a band gap energy of 1.3 eV is about 31%.
One of the processes that limits the efficiency of the conversion of solar energy into electricity in PV devices is thermalization loss due to the electron-hole pair quickly losing energy to the creation of phonons. Another loss process is recombination of the electron-hole pairs, which can be reduced by using photovoltaic materials with high lifetimes for the photo-generated carriers, or by reducing the path length of the carrier to the p-n junction.
Thermalization loss can be largely eliminated if the energy of the absorbed photon is slightly higher than the band gap energy. For example, in the multijunction solar cell concept, the solar cell includes multiple p-n junctions in a stack that are each utilized to convert a narrow range of photon energies closest to its band gap energy with the highest band gap energy material being placed uppermost towards the light source and the remaining junctions being placed in order of decreasing band gap. This design allows less energetic photons to pass through the upper layers and be absorbed in the lower layers. An infinite stack of independent cells has a calculated maximum conversion efficiency of about 67% for a non-concentrated solar spectrum. Commercial multijunction cells are designed with matched output currents so that they can be connected in series. While triple-junction solar cells have been developed with efficiencies approaching 30%, the required matching of lattice parameters and output current makes adding more junctions complex and prohibitively expensive.
Examples of nanostructured PV devices are described in U.S. Pat. No. 6,946,597 by Sager et al. and U.S. Pat. No. 7,462,774 by Roscheisen et al. Sager et al. describes a solar cell that includes a nanoporous medium and a conductive layer surrounding a polymeric light absorber, where the light absorber also acts as the conductor. In Roscheisen et al., an additional semiconductive layer is applied between the conductor and the polymer.
Another solar energy conversion approach involves using sunlight to split water into its component elements, oxygen and hydrogen, by means of photoelectrochemical cells. Several challenges remain to making this a viable technology. For example, most semiconductor materials are not durable in an aqueous environment, thus eliminating them as candidates for use in water-splitting. Single-element semiconductors, such as silicon and germanium, oxidize rapidly in water to form insulating oxides. Similarly, Group III-V semiconductors, such as the nitrides, are unstable in water and will rapidly oxidize. The only known semiconductors that are stable in water over long time periods are oxide-based semiconductors.
One of the most widely studied photoelectrochemical materials is titanium dioxide (TiO2), which has been shown to split water. TiO2 has excellent chemical stability when illuminated in an aqueous environment, and it has band edge energies well aligned with the H2 and O2 evolution reactions; however, its large band gap energy (3.0-3.2 eV) precludes absorption of most of the solar spectrum other than UV, thus limiting its efficiency. Attempts at using other oxides with smaller band gaps have been largely unsuccessful. For example, Fe2O3 has a band gap energy of 2.1 eV and is therefore ideal for water-splitting. However, iron has been shown to leach from the surface over time and the material therefore corrodes. While recent advances in WO3 production show promise, its band gap energy of 2.6 eV is too large for efficient hydrogen production.
Recently, attempts have been made to modify the band gap of TiO2 by doping it with other materials, and doping TiO2 with nitrogen and with carbon has led to optical absorption at wavelengths as low as about 500 nm, corresponding to a band gap energy of about 2.3 eV. Furthermore, the carbon-doped TiO2 showed a maximum photoconversion efficiency of 8.35% at an applied potential of 0.30 V.
Beyond their large band gaps, one of the key limitations to using oxide semiconductors such as TiO2 for photoelectrochemical materials are losses related to the short exciton diffusion length, i.e., the path length for the exciton to recombine.
Similar limitations are also inherent to particle or radiation detectors. In these devices, a large absorption cross-section is required to absorb a charged particle, but a thin semiconductor is desired to reduce recombination losses of the exciton.
Neutrons are uncharged particles, thus they cannot be detected directly. In order to detect fast neutrons, such as the prompt neutrons released during active interrogation, detectors commonly employ elastic neutron scattering by hydrogen as a detection mechanism. The scattering transfers a portion of the neutron kinetic energy to the hydrogen nucleus resulting in a recoil proton. The fraction of energy transferred ranges between zero and the full neutron energy and the average recoil proton has half of the original neutron energy. This allows for the preferential detection of fast neutrons in the presence of gamma-rays. With techniques such as pulse shape or rise time discrimination to eliminate gamma-ray events, proton recoil detectors can be used to detect a neutron energy as low as 1 keV. Recoil detectors are insensitive to thermal neutrons.
Alternatively, thermal neutrons are detectable through secondary charged particles (e.g., alpha particles) generated when they are absorbed by specific converter materials. A well-known example of a converter material is 10B, which is an alpha-converter with a thermal neutron microscopic absorption cross-section of 3840 barns. The cross-section is significantly lower for high energy, fast neutrons; however, a moderator such as high density polyethylene can be used to reduce the energy of the neutrons to the optimal detection energy. Due to its large absorption cross-section, 10B has been widely investigated as a neutron conversion material. 10B is a naturally occurring isotope with an abundance of about 20%. Thermal neutrons react with 10B by the following reaction.
                             5        10            ⁢      B        +                           0        1            ⁢      n        →      {                                        6            ⁢            %            ⁢                          :                                                                                                                               3                  7                                ⁢                Li                            ⁡                              (                                  1.015                  ⁢                                                                          ⁢                  MeV                                )                                      +                                                                               2                  4                                ⁢                He                            ⁡                              (                                  1.777                  ⁢                                                                          ⁢                  MeV                                )                                                                                      94            ⁢            %            ⁢                          :                                                                                                                               3                  7                                ⁢                Li                            ⁡                              (                                  840                  ⁢                                                                          ⁢                  keV                                )                                      +                                                                               2                  4                                ⁢                He                            ⁡                              (                                  1.470                  ⁢                                                                          ⁢                  MeV                                )                                      +                          γ              ⁡                              (                                  480                  ⁢                                                                          ⁢                  keV                                )                                                        
The high-energy alpha particles created by this reaction have a propagation range in 10B of only about 3.3 μm.
The alpha particles generated in the above reaction are detectable if they interact with a semiconductor p-n junction, where they create electron-hole pairs inside the junction, which are swept into an external circuit. Due to the large energies of the generated alpha particles, many electron-hole pairs are created. For example, if the alpha-particle deposits 1 MeV of energy into silicon (band gap energy of 3.6 eV), approximately 300,000 electron-hole pairs will be created. Conversion of a single neutron into an amplified current signal is the principle on which detectors are designed.
Unfortunately, solid state neutron detectors are inherently sensitive to gamma radiation. Gamma radiation is capable of penetrating many materials and thus is able to reach the depletion zone of the p-n junction, where it can be absorbed to create an electron-hole pair. Since this is the same mechanism by which neutrons are detected, gamma radiation can cause false positives. Comparing the detector signal with the signal from a second reference device, which does not contain the neutron converter material, normally eliminates these false positives.
Traditionally, solid state neutron detectors are fabricated in a planar configuration by coating a layer of 10B-based alpha-converter material onto a planar semiconductor. These devices have limited efficiency due to the conflicting thickness requirements of the converter material. The converter material must be thick enough to capture all the incoming neutrons, while at the same time being thin enough to allow the alpha particles to reach the semiconductor.
Nikolic et al. have proposed that these conflicting thickness requirements can be eliminated by changing the device architecture from a planar configuration to a 3-dimensional configuration. In this regard, FIG. 1 illustrates a proposed 3-dimensional configuration for a solid-state neutron detector. The solid-state neutron detector 100 includes rods 102 of boron disposed within a p-type semiconductor layer 104 and an intrinsic semiconductor layer 108. An n-type semiconductor layer 106 is used to form a backside ohmic contact. Metal layers 110 and 112 form electrical contacts to complete the structure.
Monte Carlo simulations of detectors with a 3-dimensional architecture have been performed and suggest that much higher detection efficiencies can be achieved. One architecture that was modeled consisted of a semiconducting material with an array of etched holes filled with 10B. The simulations showed that the largest detection efficiency was obtained for the smallest cell dimension that was modeled (2 μm). The simulations also showed that efficiency increased as the fraction of the cell occupied by 10B increases. Another architecture that was modeled consisted of an array of semiconducting pillars embedded in a 10B matrix. These simulations showed that as the pillar diameter and spacing was decreased, the detection efficiency increased. For example, 50 μm tall, 100 nm diameter pillars with a 100 nm separation can give a detector efficiency near 90%. The simulations also suggest that as the pillar size is further reduced below 100 nm, the detector efficiency can approach 100%.
While a 3-dimensional neutron detector such as that illustrated in FIG. 1 is expected to show dramatic improvements compared to a planar detector, a number of performance limitations still exist. First, the semiconductor layers must be at least as thick as the 10B layer (i.e., tens of μm). Since the minority carriers created by the alpha particle must travel through the thick semiconductor layer to reach the electrode, the semiconductor must be of high quality (i.e., high cost) in order to avoid recombination losses. However, exposure to ionizing radiation damages semiconducting materials, and consequently the performance of the device can be expected to degrade over time. Second, in order to obtain the highest sensitivity, the aspect ratio of the 10B pillars should be 500 or greater. This aspect ratio is very challenging to fabricate in silicon by micromachining, leading to high production costs. Third, the thickness of the depletion layer must be comparable to the thickness of the 10B layer, and therefore the bias voltage must be large, leading to large power consumption. The larger bias voltage will limit the utility of the solid state neutron detectors for low power applications, such as shipping container monitoring.
The foregoing energy conversion devices function by taking an energy input (e.g., light, radiation) and converting it to an electrical current; however, the opposite process is governed by similar architectural constraints. A light emitting diode (LED) is a semiconductor diode that emits light when the p-n junction is forward biased. In LEDs the light emits within a narrow spectrum, which produces the distinct colors seen for LEDs. Initially restricted to red light, and later extended to yellow, shorter wavelength LEDs have become available, such as blue LEDs based on GaN and InGaN.
Recently, LEDs have been fabricated based on AlGaN and AlInGaN with wavelengths extending below 230 nm, potentially enabling deep UV LEDs with wavelengths below 280 nm and high quantum efficiencies. These LEDs are all based on multiple quantum well (MQW) structures. MQWs are layers of slightly different materials deposited one on top of the other. For example, a 5-period AlN/Al0.85Ga0.15N with a well thickness of 20 Å was shown to emit light at about 230 nm. MQW structures such as these have unequivocally demonstrated the path to generate deep UV from LEDs.